The present invention relates to microwave frequency devices and methods of fabricating same.
Microwave frequency components, including surface mount components, are increasingly being used to provide transmission lines and other circuit functions that are useful to designers of larger systems. Strip line and microstrip techniques are often used to implement these microwave frequency devices.
The microstrip technique is characterized by a planar transmission line conductor disposed on a dielectric layer and spaced apart from a conducting ground plane. This construction establishes an impedance and a velocity factor of the transmission line, which are functions of such factors as the dielectric characteristics of the dielectric layer and other surrounding materials, a width of the planar transmission line conductor, and the distance from the planar transmission line conductor to the conductive ground plane.
The strip line technique is generally characterized by a planar transmission line conductor sandwiched between two dielectric layers and between two conductive ground planes on opposite sides of the dielectric layers. This construction provides a shield around the planar transmission line vis-à-vis the two conductive ground planes that sandwich the transmission line. This construction also establishes an impedance and a velocity factor of the transmission line, which are functions of such factors as the dielectric characteristics of the dielectric layer and other surrounding materials, a width of the planar transmission line conductor, and the distance from the planar transmission line conductor to the conductive ground planes.
Among the concerns of a designer of microwave frequency devices and larger systems in which such devices are utilized, are the mechanisms by which microwave signals are input to and output from the microwave frequency devices. For example, a microwave frequency device (such as a directional coupler, a power divider, etc.) fabricated utilizing strip line technology may be part of an overall system containing other components. Interconnections between the directional coupler and other devices of the system may be made by way of a printed circuit board (PCB), where connecting traces are formed utilizing the microstrip technique. Under these circumstances, the planar transmission line conductors of the microwave frequency devices of the system are electrically connected to the traces of the printed circuit board.
U.S. Pat. No. 4,821,007 (“the '007 patent”) provides an illustrative example of the electrical interconnections between a strip line microwave frequency device that is surface mounted to a printed circuit board. The '007 patent is hereby incorporated by reference in its entirety. In accordance with the '007 patent, the electrical connections between the planar transmission line conductors of the strip line microwave frequency device and the traces of the printed circuit board are made by way of portions of plated through-holes passing through a laminar assembly. The plated through-holes are bisected during the manufacturing process to expose the portions of the plated through-holes at a peripheral edge of the structure.
More particularly, the laminar assembly disclosed in the '007 patent includes one or more planar transmission lines sandwiched between two dielectric layers and two outer ground planes disposed on opposite sides of the dielectric layers. A series of holes are drilled through the laminar assembly (i.e., through the two dielectric layers) such that they intersect the planar transmission lines. The through-holes are then plated such that an electrical connection is made between the plating and the planar transmission lines. The laminar assembly is then cut along lines that bisect the through-holes such that portions of the plated through-holes are exposed. The planar transmission lines of the laminar assembly are electrically connected to the traces of the printed circuit board by soldering the plating of the exposed through-holes to the traces.
Unfortunately, plated through-holes are notoriously unreliable and often fail. Indeed, as the number of layers through which a through-hole passes increases, the reliability of the through-hole decreases exponentially. Therefore, the connection of a multi-layer microwave frequency device to a printed circuit board utilizing an exposed plated through-hole as described in the '007 patent presents a problem. Indeed, the transfer of a microwave signal from the microwave frequency device to the printed circuit board, or vice versa, may not be reliable. Further, abrupt changes in geometry from a planar transmission line of a microwave frequency device, to the plated portion of an associated multi-layer through-hole, and to a trace of a printed circuit board, are prone to produce impedance mismatches and resultant undesirable signal reflections.
Still further, the use of the strip line technique in signal transmission has an inherent limitation on power handling capability inasmuch as the widths of the planar transmission lines are relatively small for a given impedance. Indeed, a plated through-hole (like that used in the '007 patent) may be of about 50 mils (0.050 inches) in diameter, while the planar transmission line may be about 10 mils (0.010 inches) wide. Mismatches caused by radical geometry changes at the plated through-hole to PCB junction will cause high temperatures at the planar transmission line. Since the planar transmission line is only 10 mils wide, it might fuse. Therefore, maintaining a strip line construction within a microwave frequency device to the interconnection of the planar transmission lines and the traces of the printed circuit board limits the power handling capability of the device, particularly at the interconnection points.
While impedance mismatching can sometimes be compensated for by tuning techniques (e.g., adding capacitance or inductance at key positions in the circuit), the construction of the '007 patent does not provide for such action on the microwave frequency device. Employing tuning techniques on the PCB is not a practical solution because system manufacturers expect that the device to operate “as advertised” without requiring tuning after assembly to the PCB.
Accordingly, there are needs in the art for new microwave frequency devices, and methods of manufacturing same, which provide different mechanisms for interconnecting the microwave frequency devices to the traces of a printed circuit board, preferably mechanisms that enjoy enhanced power handling capability and the ability to tune the signal lines at the interconnection point to adjust for impedance mismatches and reduce signal reflections.